The invention relates to electric switches in general, and more particularly to improvements in snap action switches. Still more particularly, the invention relates to improvements in snap action switches with switching units wherein the contact (also called mainplate) which must be caused to move from a first end position toward and through a dead-center position in order to be capable of snapping to a second end position has a deformable blade within a frame which is or which can be affixed to a mounting member of the switching unit.
As a rule, the mainplate of the switching unit in a snap action switch has a substantially rectangular frame and a center member or blade which is parallel with the longer sections (first outside members) and has end portions integral with the shorter sections (second outside members)of the frame. The central portion of each longer section of the frame is provided with a V-shaped or omega-shaped corrugation which entails a reduction of the length of such sections and causes a corresponding deformation (bulging) of the blade. The four corners of the frame are provided with bent-over extensions which are anchored in the bottom wall of the mounting member in the switching unit. This ensures that the four corners of the frame remain in positions of abutment with the adjacent side of the bottom wall irrespective of the extent of deformation of the blade. In other words, the mainplate is installed in stressed condition in order to ensure that the median portion of the blade will invariably tend to assume a predetermined (first) end position in which it is normally remote from a contact to be engaged by the median portion of the blade in order to complete an electric circuit. Two of the aforementioned extensions (normally at two diagonally opposite corners of the frame) are sufficiently long to project through and beyond the bottom wall of the mounting member and to serve as terminals which are connected to conductor means. When the median portion of the blade is caused to assume its second end position, it engages the aforementioned contact whereby the conductive mainplate completes a circuit by enabling electric current to flow from the two longer extensions, through the frame and through the blade and on to the contact which is engaged by the median portion of the blade. The contact for the median portion of the blade can be a flat or slightly arched piece of conductive metallic material, and a portion of this contact also extends through the bottom wall to be connected with a conductor.
The deformation of the blade in the first end position of its median portion is normally such that a concave side of the blade faces toward the contact on the bottom wall of the mounting member. The first end position of the median portion of the blade is a stable position, i.e., the median portion tends to assume and to thereupon remain in the first end position. The means for moving the median portion of the blade from its first end position comprises an actuator which is movable relative to the mounting member in order to force the median portion toward the aligned contact on the bottom wall of the mounting member. The second end position of the median portion of the blade is preferably an unstable position, i.e., the median portion remains in the second end position only as long as it is acted upon by a force which suffices to prevent it from reassuming the first end position due to its innate resiliency and due to stressing of the frame of the mainplate.
The just discussed mainplate and the switching unit which employs such mainplate exhibit two serious drawbacks. First of all, each movement of median portion of the blade to its second end position necessitates very pronounced stressing of the entire mainplate so that the mainplate, and especially its frame, is likely to break after a relatively small number of actuations of the switching unit. The reason for such pronounced stressing of the frame is that all four corners of the frame are anchored in the mounting member of the switching unit, i.e., the frame cannot yield during movement of the median portion of the blade from its first end position toward and beyond the dead-center position. Attempts to overcome such drawbacks of conventional mainplates include anchoring of two of four extensions of the frame with a certain amount of play relative to the mounting member; however, this offers very little relief as far as the stressing of the frame is concerned. Secondly, the switching characteristics and the so-called touch of a switching unit which employs a mainplate wherein all four corners of the frame are anchored in the mounting member are far from satisfactory. Thus, the magnitude of the force which is needed to move the median portion of the blade from its first end position varies very little so that the operator in charge of effecting actuation of the switching unit cannot readily discern that stage when the median portion of the blade moves through the dead-center position. In addition, it is necessary to exert a rather pronounced force in order to maintain the median portion of the blade in the (unstable) second end position, i.e., in that end position in which the median portion of the blade remains only as long as it is acted upon by a substantial external force.